I trade ETF's for two reasons; either

a) it's easier than directly trading the commodity eg. DBA, IAU

b) it spreads the risk versus individual stocks in a risky sector eg. IBB (biotech).

I trade and/or invest in plenty of individual biotech stocks but after much discussion with my wife, decided not to convert my office into a soy bean silo!

Btw - tell me DBA is not an absolutely beautiful chart and you wish you'd owned it!! (bought in early october - took some off the table already).
Granted you could have played some agg stocks for the same or better gain but this is the pure play on the commodities.